1. Field of the Invention
The present invention relates to optical systems and more particularly to a method and apparatus for testing the optical quality of aspherical mirrors. The invention further relates to an optical technique for mapping the surface defects in spherical or non-circularly symmetric mirrors.
2. Related Technical Art
High quality reflective mirrors are used in a variety of advanced optical applications such as image scanners or sensors, image duplication, or image digitizing systems. In high resolution optical systems, the mirrors must possess high quality optical surfaces and be very accurately aligned to provide a desired high resolution or high image registration. Therefore, it is necessary to provide methods of manufacturing mirrors with very precisely defined focal patterns with extremely minimal zonal or surface defects, and that are capable of high accuracy alignment within a given optical system. The alignment process may include system alignment, but also includes initial alignment of the mirror within a mounting structure for later use.
When flat, circular, or circularly symmetric mirrors are employed it is often simple to align the mirrors by simply using a mechanical measurement of the focal point of the mirror. The size and circular curvature of such mirrors are easy to measure, and from this the focal point or axis is also easily determined. Light beams can also be reflected from the mirror surface and the reflected light observed to form a common intersection point which is used to trace predetermined patterns. Another method is to rotate the mirror while reflecting a collimated light beam or an image from its surface and adjusting the mirror's axis of rotation until a reflected image is stationary. A variety of interferometric techniques are also available using incident and reflected laser light beams and images. However, when dealing with aspherical or parabolic shapes or more complex geometries and offset mirrors, the traditional interferometric methods do not work well.
Many of the more complex aspherical mirrors are manufactured to high tolerances using special diamond turning tools and polishing processes. Unfortunately, these tools and techniques often leave small "tooling" marks or surface imperfections and degradations, which create excessive light scatter or spurious reflections and make the use of high resolution interferometric laser apparatus impossible. At the same time, zonal defects in some aspheric mirrors are of such a size or irregularity in nature that interferometry techniques are severely impacted and often useless.
One method of mirror alignment is to assume that any zonal error is concentric or symmetric about the central optical axis if alignment is perfect. An interferogram is used to detect or focus to a mirror zone, which is then used to establish the optical axis. Likewise, Ronchi tests can be used to find a pattern passing a mirror zone. However, with tooling marks and other surface errors, the zonal symmetry assumption proves to be incorrect and the alignment techniques fail, unless additional information about the surface variations can be provided.
Aspherical mirrors and mirrors having complex surface shapes are required for many advanced optical systems. Therefore, what is needed is a method and apparatus for providing high accuracy measurement of the optical quality and focal characteristics of such mirrors. It is also desirable for any optical technique to be usable with a minimum of expense and complication, while being easily automated for more efficient mirror processing.